Avoid crashing when localhost attempts to trigger COOP isolation.

ChildProcessSecurityPolicyImpl::AddIsolatedOriginForBrowsingInstance()
assumes that it only ever seems valid isolated origins and uses a
CHECK to enforce this.  Callers of this function that use legacy
isolated origins (i.e., not Origin-Agent-Cluster) are expected to
check IsolatedOriginUtil::IsValidIsolatedOrigin() on the origin being
isolated prior to calling this.

Android's COOP-triggered site isolation mode (controlled by
features::kSiteIsolationForCrossOriginOpenerPolicy, enabled by default
in M95) has a couple of code paths that lead to calling this function.
After seeing a user gesture, it uses SiteInstance::StartIsolatingSite,
which has a IsValidIsolatedOrigin check.  But prior to seeing a user
gesture, SiteInstanceImpl::SetSiteInfoInternal() calls
AddIsolatedOriginForBrowsingInstance directly, gated by
site_info_.does_site_request_dedicated_process_for_coop(),and nothing
leading up to this path checks IsValidIsolatedOrigin.  This CL fixes
that by adding a check for IsValidIsolatedOrigin in
NavigationRequest::ShouldRequestSiteIsolationForCOOP(), which is
where the COOP isolation plumbing starts.

Bug: 1276155
Change-Id: I465ce9cd99e2f45d306bf0031f9e4f0174f5b1a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3313961
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#948697}